Strange digital noise in SAR ADC

[Circuit_seller]

Strange Noise

Hi

I have used AD8138 to convert a single-ended signal to differential one and fed its output to AD7687( this ADC accepts differential analog inputs).
To read samples serialy from this ADC i have used my DSP McBSP.
I saw a strange problem! when i have no signal at the input of the AD8138 i see a strange digital noise converted by the ADC( you can see this noise at the picture).
I checked the input of the ADC with scope and saw that one of the inputs of the ADC hase this strabge signal.
Does anyone have any idea about the source of this noise?
I should mention that the type of this ADC is SAR .

Regards

I do not know why we can not see the picture.

 

[Verba]

Strange Noise

i guess schematic would be helpful.
may be you just have your clock coupled to the input ?
what do you mean by "no signal" ? input to the ground or not connected ?

 

[Circuit_seller]

Strange Noise

Hi

The input is connected to gnd via a resistor, and about clock coupling if you watch the picture you see that this signal is very strange and does not have any correlation with the clock.

Regards